The optical system of the human eye uses numerous muscles as well as central and peripheral cues while focusing on targets both near and far. There are many responses involved in changing focus from distant to near as well as fixating on a target at a set distance.
When our eyes are working together and are directed at a target greater than twenty feet from our eyes they will appear to be parallel with each other and we deem this binocularity. If both eyes are looking at a target closer than twenty feet our eyes may not look parallel but we a still have binocular vision as long as the line of sight of each eye is pointing directly at the target of regard. If binocularity is canceled by interrupting the vision of one eye or the other the eyes often rotate along X, Y, & Z-axis. The movement and rotation of the eye that is covered compared to the movement of the eye that is uncovered may be different, but measurable. In general terms the change in position or movement of the eyes once vision is interrupted, ending binocularity, is often deemed a heterophoria. It is also possible to measure the torsional rotation and movement along the X, Y, & Z axis of the eye by not interrupting one eye or disrupting binocularity. This may be done by altering the position of peripheral binocular targets located in a relationship to the central binocular targets.
Our proprioceptive system, or what we often call our “sixth sense”, is the sensory feedback mechanism for motor control and posture. It gives us unconscious feedback internally. Our proprioceptive system is composed of sensory neurons located in our inner ear and stretch receptors located in our muscles and supporting ligaments.
In our skeletal muscles these proprioceptive receptors have a load compensating mechanism. For example: imagine standing with eyes closed and arms extended outward. Now imagine someone starting to load one book after another on your hands. As you feel the weight of the books increase, you exert more force in order to keep the books from falling to the ground. When maximum effort is reached the books will fall from your hands. You do not need your eyes to sense the weight.
There are anatomically similar proprioceptive receptors in our ocular muscles but these receptors do not have a load compensating mechanism and do not mediate conscious eye position. This is understandable because there is a constant mechanical load on all the extraocular muscles and no load compensating mechanism is required.
Our extraocular muscles have proprioceptive receptors that constantly give feedback to the location of each eye. When we choose to look at something our brain takes the image from each eye and moves our extraocular muscles to exactly line up to the target. If this did not happen you would have blurred vision one eye pointing at one target and the other eye pointing at a different target.
You can choose where you want to look but then your autonomic nervous system takes the image from each eye and sends a signal to your extraocular muscles to line each eye up perfectly at that target. After the movement of each eye independently to line up the target the proprioceptive receptors in your extraocular muscles send the signal back to brain as to the position of where each eyes has been moved to. This proprioceptive feedback is necessary to close the loop between where the brain told the eyes to move and where the eyes are currently located. The brain needs to know the position of each eye so that when you decide to look at the next target your brain knows how much to move each eye in order to line up to the next target.
This proprioceptive feedback is critical for coordinating the movements between our eyes, seeing a single clear image, along with many other functions. We know that this proprioceptive feedback from our extraocular muscles sends its signal via the trigeminal nerve, which is a nerve in our head responsible for pain sensation in our sinuses, extraocular muscle tissue, and jaw.
Many people who suffer from chronic headaches, asthenopia associated with near work, asthenopia associated with viewing distance targets, stiff neck and shoulder muscles, and dry eyes are the consequence of the extra ocular muscles proprioceptive sensory feedback mechanism stimulating the trigeminal nerve. From clinical study with chronic headache patients we have learned that changing this feedback loop can alter and often alleviate headache pain. This can be done by measuring proprioceptive disparity or more generally visual fixation disparity. Proprioceptive disparity is the imbalance between where the eyes are consciously focused and the nonvisual perception of where the object is located in space. This often varies with distance.
Testing and synchronizing the proprioceptive feedback between each extraocular muscle requires isolating our central vision from our peripheral vision. Our central vision sustains less than 1° of arc and is responsible for detailed vision located within the area of our retina called our fovea. Targets seen in the fovea are controlled by slow smooth pursuits eye movements. Targets outside of our fovea and in our peripheral vision are controlled by quick saccadic eye movements. Anatomically we know that pursuits and saccadic eye movements are coordinated in our brain from different locations.
The use of electronic image capture devices to observe and quantify the movement of the human eye is a mature technology known as “eye tracking”. Some applications of eye tracking include military equipment for pilots, sophisticated 3-D virtual reality environments, and medical analysis.
Good quality stereo 3-D display technology is relatively new to consumer products, but has been available for professional applications for many years. A variety of 3-D display technologies have been developed which endeavor to provide the viewer with two visual images, one for each eye, which differ slightly in their content so as to present all targets in the visual field with their mathematically correct parallax according to distance from the viewer. The oldest movie technology used different glasses with colored filters for each eye. This was crude and unrealistic. Current technology for movies uses glasses with either passive polarized filters or active-shutter electronics. New technologies for single user displays are autostereoscopic (i.e., not requiring glasses) and incorporate lenticular lenses or parallax barriers to provide separate images for each eye.
This application is directed to improvements in testing proprioceptive feedback.